Torrey Lindbo, Jan Miller, SWRP Clean Water Services Water Quality Index Torrey Lindbo, Jan Miller, SWRP Clean Water Services

Role of the Water Quality Index WQI numerically summarizes the information from multiple water quality parameters into a single value The single value can be used to compare data from several sites It can be used to look at trends over time on a single site

Parameters used for WQI Units For a Q value of 90 [Oregon standards] DO % sat. % sat. >82% [>90% unless spawning, then >95%] pH pH units 6.9 or 7.9 [6.5 to 8.5] E. coli CFU/100 mL <4 CFU/100 mL [406 CFU/100 mL for grab sample]  Temp  °C +2.5°C or -4°C  [<17.8 deg. C] Turbidity NTU 4.0 NTU’s T Phos mg/L P 0.05 mg/L P [0.1 mg/L P, Tualatin TMDL] NO3 mg/L N 0.5 mg/L N BOD5 mg/L 0.8 mg/L Student Watershed Research Project (SWRP)

Parts of the Water Quality Index Q-Value - indication of water quality relative to 100 of one parameter Weighting Factor - sets the relative importance of the parameter to overall water quality

Q Value The Q-Value is an indication of how good (or bad) the water quality is relative to one parameter. 100 = Very Good 1 = Very Bad Q = 0 Q = 100 Q = 25 Q = 75 Q = 50 Very Bad Very Good Medium

pH Q-Graph and Q-Table pH Q-Value 6.6 81 6.8 88 7.0 94 7.2 98 7.4 100 6.6 81 6.8 88 7.0 94 7.2 98 7.4 100 7.6 98 7.8 94 8.0 88 8.2 80 8.4 72

E.coli Q-Graph & Q-Table E. coli Q-Value 0 100 1 99 2 94 5 89 10 84 20 80 50 74 100 69 200 65 500 27 1000 30

Weighting Factors DO 0.18 pH 0.12 E. coli 0.17 Temp 0.11 Turbidity 0.09 T Phos NO3 0.10 BOD Total 1.00

Water Quality Index Range WQI Interpretation Water Quality Index Range Water Quality Rating 90-100 Excellent 70-89 Good 50-69 Medium 25-49 Bad 0-24 Very Bad

Example WQI Calculation Parameter Units Result Q-Value Weight Factor Parameter Index DO % Sat % 95 98 0.17 17.67 pH pH units 8 88 0.11 10.55 E. coli CFU/100 mL 1750 23 0.16 3.94 Temp  deg C 100 11.00 Turbidity NTU 10 79 0.09 7.07 T Phos mg/L P 0.2 50 0.1 5.50 NO3 mg/L N 0.4 91 9.12 BOD mg/L 6 36 4.34 Total 1.00 69.2 WQI = Quality Rating = MEDIUM

Adjusting for Missing Data When a parameter is missing, the sum of the weighting factors that have data can be used to adjust the WQI Sum of the Parametric Index Values is divided by the sum of the weighting factors that have data.

Calculation with Missing Parameter Units Result Q-Value Weight Factor Parameter Index DO % Sat % 95 98 0.17 17.67 pH pH units 8 88 0.11 10.55 E. coli CFU/100 mL 1750 23 0.16 3.94 Temp  deg C 100 11.00 Turbidity NTU 10 79 0.09 7.07 T Phos mg/L P NM NO3 mg/L N 0.4 91 0.1 9.12 BOD mg/L 6 36 4.34 Total 0.89 63.7 WQI = 71.6 Quality Rating = GOOD